class_docstr: GRU cell for matrix, similar to the official code.
  Please refer to section 3.4 of the paper for the formula.
code: def forward(self, prev_Q, z_topk=None):
      if z_topk is None:
          z_topk = prev_Q
  
      update = self.update(z_topk, prev_Q)
      reset = self.reset(z_topk, prev_Q)
  
      h_cap = reset * prev_Q
      h_cap = self.htilda(z_topk, h_cap)
  
      new_Q = (1 - update) * prev_Q + update * h_cap
  
      return new_Q
  
imports: import torch
  import torch.nn
  from dgl.nn.pytorch import GraphConv
  from torch.nn import init
  from torch.nn.parameter import Parameter
label_desc: Use image node_id 2 for calling the MatGRUCell obj's underlying member method code with example usage: obj.forward(prev_Q, z_topk) and returns: new_Q